In numerous installations for processing chemical products it is necessary to have available both steam and a hot gas employed generally in the reactions. The steam is employed for supplying heat to the chemical reactions or for heating certain products. It may serve for feeding steam ejectors. It also serves for feeding a steam turbine which supplies mechanical or electrical energy.
Thus, in a unit for refining petroleum products the units for fractional distillation under pressure or under vacuum and the units for catalytic cracking or for catalytic reforming necessitate the application of heat, and require that the products which enter be heated. In a unit for atmospheric distillation, for example, the crude petroleum, after having been preheated, is heated to a temperature lying between 350.degree. and 370.degree. C. The chain of catalytic reforming operates at a temperature in the range460.degree. and 580.degree.. Present refineries are equipped, in addition to the boiler or boilers supplying the steam, with process furnaces which serve to heat the products before or during the reactions or processing.
It has been proposed to employ a steam boiler operating at a pressure higher than 120 bars for feeding the distillation columns which play the part of condensers. The condensation of steam at high pressure and high temperature assures the distillation of the crude. Another steam circuit at high pressure feeds a back-pressure turbine in which the steam is expanded to different pressure levels corresponding to different levels of condensation temperatures.
This steam, expanded to different extents, is employed for heating, for example for the heating of the crude petroleum before its entry into the columns. This method enables substantial improvement in the energy balance of the refinery. However, the gases employed in the chain of catalytic reforming are heated in the process furnaces, which leads to certain disadvantages. These furnaces are very inefficient. Furthermore, the temperature of the gas feeding the catalytic reforming may vary as a function of the accidental overheating which is inevitable in a furnace. Finally, the dispersion of furnaces and boilers about the refinery complicates the operation and has the effect that the area occupied by the thermal equipment is large.